In the recent drive for higher integration densities and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in 1980's. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) in 1990's and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with the mass-scale fabrication of 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the development of F2 lithography was abandoned and instead, the ArF immersion lithography was introduced.
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water having a refractive index of 1.44. The partial fill system is compliant with high-speed scanning and when combined with a lens having a NA of 1.3, enables mass production of 45-nm node devices.
One candidate for the 32-nm node lithography is lithography using extreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUV lithography has many accumulative problems to be overcome, including increased laser output, increased sensitivity, increased resolution and minimized line edge or width roughness (LER, LWR) of resist film, defect-free MoSi laminate mask, reduced aberration of reflection mirror, and the like.
Another candidate for the 32-nm node lithography is high refractive index liquid immersion lithography. The development of this technology was abandoned because LUAG, a high refractive index lens candidate had a low transmittance and the refractive index of liquid did not reach the goal of 1.8.
The process that now draws attention under the above-discussed circumstances is a double patterning process involving a first set of exposure and development to form a first pattern and a second set of exposure and development to form a pattern between the first pattern features. A number of double patterning processes are proposed. One exemplary process involves a first set of exposure and development to form a resist pattern having lines and spaces at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying another layer of hard mask thereon, a second set of exposure and development of a resist film to form a line pattern in the spaces of the first exposure, and processing the hard mask by dry etching, thereby forming a line-and-space pattern at a half pitch of the first pattern. An alternative process involves a first set of exposure and development to form a resist pattern having spaces and lines at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying a resist layer thereon, a second set of exposure and development to form a second space pattern on the remaining hard mask portion, and processing the hard mask by dry etching. In either process, the hard mask is processed by two dry etchings.
As compared with the line pattern, the hole pattern is difficult to reduce the feature size. In order for the prior art method to form fine holes, an attempt is made to form fine holes by under-exposure of a positive resist film combined with a hole pattern mask. This, however, results in the exposure margin being extremely narrowed. It is then proposed to form holes of greater size, followed by thermal flow or RELACS® method to shrink the holes as developed. With the hole shrinking method, the hole size can be shrunk, but the pitch cannot be narrowed.
It is then proposed in Non-Patent Document 1 that a pattern of X-direction lines is formed in a positive resist film using dipole illumination, the resist pattern is cured, another resist material is coated thereon, and a pattern of Y-direction lines is formed in the other resist film using dipole illumination, leaving a lattice-like line pattern, interstices of which provide a hole pattern. Although a hole pattern can be formed at a wide margin by combining X and Y lines and using dipole illumination featuring a high contrast, it is difficult to etch vertically staged line patterns at a high dimensional accuracy. It is proposed in Non-Patent Document 2 to form a hole pattern by exposure of a negative resist film through a Levenson phase shift mask of X-direction lines combined with a Levenson phase shift mask of Y-direction lines. Undesirably, the resolving power of the crosslinking negative resist film is low as compared with the positive resist film because the maximum resolution of ultrafine holes is determined by the bridge margin.
A hole pattern resulting from a combination of two exposures of X- and Y-direction lines and subsequent image reversal into a negative pattern can be formed using a high-contrast line pattern of light. Thus holes having a narrow pitch and fine size can be opened as compared with the prior art.
Non-Patent Document 3 reports three methods for forming hole patterns via image reversal. The three methods are: method (1) involving subjecting a positive resist composition to two double-dipole exposures of X and Y lines to form a dot pattern, depositing a SiO2 film thereon by LPCVD, and effecting O2-RIE for reversal of dots into holes; method (2) involving forming a dot pattern by the same steps as in (1), but using a resist composition designed to turn alkali-soluble and solvent-insoluble upon heating, coating a phenol-base overcoat film thereon, effecting alkaline development for image reversal to form a hole pattern; and method (3) involving double dipole exposure of a positive resist composition and organic solvent development for image reversal to form holes.
The organic solvent development to form a negative pattern is a traditional technique. A resist composition comprising cyclized rubber is developed using an alkene such as xylene as the developer. An early chemically amplified resist composition comprising poly(tert-butoxycarbonyloxy-styrene) is developed with anisole as the developer to form a negative pattern.
Recently a highlight is put on the organic solvent development again. It would be desirable if a very fine hole pattern, which is not achievable with the positive tone, is resolvable through negative tone exposure. To this end, a positive resist composition featuring a high resolution is subjected to organic solvent development to form a negative pattern. An attempt to double a resolution by combining two developments, alkaline development and organic solvent development is under study.
As the ArF resist composition for negative tone development with organic solvent, positive ArF resist compositions of the prior art design may be used. Such pattern forming processes are described in Patent Documents 1 to 7.
These Patent Documents disclose resist compositions for organic solvent development comprising polymers having copolymerized therein hydroxyadamantane methacrylate, norbornane lactone methacrylate, or methacrylates having an acidic group such as carboxyl, sulfo, phenol or thiol substituted with an acid labile group as the base resin, and pattern forming processes using the compositions.
The methacrylate resins, however, have the drawback that the resin backbone is short of dry etch resistance. When the substrate is etched using the resist film as a mask, there is a likelihood of pattern transfer failure. It is known from Non-Patent Document 4 that dry etch resistance can be effectively improved by introducing an alicyclic structure. Then many attempts were made to introduce an alicyclic structure into a pendant chain of methacrylate resins. In the positive pattern forming process involving alkaline development, some fruitful results are obtained by introducing an alicyclic structure, typically alkyladamantyl group into an acid labile group.
In the negative pattern forming process involving organic solvent development, on the other hand, dry etch resistance becomes a consideration again because the portion to turn insoluble during development corresponds to the portion where the acid labile group of alicyclic structure is deprotected.
Besides the methacrylate resins, Patent Document 8 proposes a pattern forming process in which a resist composition comprising a polymer having an alicyclic structure in its backbone as a base resin is combined with organic solvent development. It is expected that this polymer is somewhat improved in dry etch resistance over the methacrylate resin, although the patent document refers nowhere to dry etch resistance. However, the polymer is not regarded sufficient with respect to the resolution and roughness of a fine pattern necessary for the fabrication of advanced microelectronic devices.
In general, the negative development with organic solvent is low in dissolution contrast, as compared with the positive development with alkaline developer. Specifically, in the case of alkaline developer, the alkali dissolution rate differs more than 1,000 times between unexposed and exposed regions, whereas the difference is only about 10 times in the case of organic solvent development. Increasing the proportion of acid labile group introduced is effective for enhancing the dissolution contrast and improving the resolution, but in the case of negative development, may worsen dry etch resistance for the above-described reason.